1. Field of the Invention
This invention relates to a variation compensating unit and, more particularly, to a variation compensating unit for compensating variation including at least one of an amplitude variation and phase variation which will occur in the case of transmitting signals via transmission paths.
2. Description of the Related Art
In recent years attention has been riveted on cellular mobile communication systems in which digital signal processing is performed on signals to be transmitted or received by locating a plurality of antenna elements, such as multiple beam antennas or adaptive array antennas, at a radio base station.
FIG. 18 is a schematic view showing the structure of a system using an adaptive array antenna. FIG. 18 shows the structure of a receiving section. In this example, four antennas 101a through 101d are located on transmission paths respectively. On the first transmission path, a low-noise amplifier (LNA) 102a, frequency converters 103 and 105, an amplifier 104a, an A/D converter 106a, a multiplier 107a, and a combining section 108 are located. On the second transmission path, a low-noise amplifier (LNA) 102b, the frequency converters 103 and 105, an amplifier 104b, an A/D converter 106b, a multiplier 107b, and the combining section 108 are located. On the third transmission path, a low-noise amplifier (LNA) 102c, the frequency converters 103 and 105, an amplifier 104c, an A/D converter 106c, a multiplier 107c, and the combining section 108 are located. On the fourth transmission path, a low-noise amplifier (LNA) 102d, the frequency converters 103 and 105, an amplifier 104d, an A/D converter 106d, a multiplier 107d, and the combining section 108 are located. The frequency converters 103 and 105 include a local oscillator (LO) and mixers.
Signals are received by the antenna 101a and are transmitted to the LNA 102a. Signals output from the LNA 102a, the amplitude of which is far higher than that of the original signals, are converted from RF signals into IF signals by the frequency converter 103. These IF signals are amplified by the amplifier 104a, are converted into base band signals by the frequency converter 105, are converted into digital signals by the A/D converter 106a, and are given weight W by the multiplier 107a. This is the same with the antennas 101b through 101d. The weighted signals are combined by the combining section 108. Received (or transmitted) signals are expressed as functions of complex variables with amplitude as a and phase as xcex8.
FIG. 19 is a view showing a beam pattern obtained by an array antenna. As shown in FIG. 18, it is assumed that radio signals which arrive from direction "psgr" to the antennas 101a through 101d are received. There arise path differences among the antennas 101a through 101d. These path differences are expressed as A1 through A3, respectively, with the antenna 101a as a standard. These path differences will lead to phase differences. The radio signals are weighted by the multipliers 107a through 107d so that these phase differences will be canceled out, and then are combined by the combining section 108. Signals output from the combining section 108 are equivalent to signals received as beam pattern B1 shown in FIG. 19.
Beam pattern B1 obtained by receiving with an adaptive array antenna and beam pattern B2 obtained by receiving with one antenna will now be compared. It is assumed that desired user signals arrive from direction "psgr", that interference user signals arrive from directionxcex7, that the levels of the desired and interference user signals received as the beam pattern B1 are P1 and P2 respectively, and that the levels of the desired and interference user signals received as the beam pattern B2 are P3 and P4 respectively. With the beam pattern B2, the difference in level (La) between P3 and P4 is small, but, with the beam pattern B1, the difference in level (Lb) between P1 and P2 is significant. Therefore, the beam pattern B1 gives greater S/I than the beam pattern B2.
That is to say, with cellular mobile communication systems in which multiple beam antennas, adaptive array antennas, or the like are used, a beam pattern equivalently becomes sharp. As a result, interference in areas can be reduced. In addition, higher gain is obtained, so the number of users who can be accommodated in one cell can be increased.
In order to realize beam forming by the above system, nonlinear elements, such as the LNAs 102a through 102d and mixers, are needed on the receiving side for converting RF signals received by the antennas 101a through 101d into base band signals, as shown in FIG. 18.
Moreover, on the transmitting side (a transmitting section is not shown), nonlinear elements, such as mixers for converting base band signals into IF signals, then into RF signals and a high power amplifier (HPA) for RF signals, are needed on each antenna branch.
However, these nonlinear elements included in each circuit differ in characteristic. Their characteristics also change according to environmental conditions, such as temperature, and input levels and suffer aged deterioration. As a result, amplitude and phase variations differ among different antenna branches, so efficient beam forming cannot be performed. This will lead to degradation in characteristic.
Compensating these amplitude and phase variations therefore is essential for the introduction of a multiple beam antenna or adaptive array antenna.
Conventionally, calibration between antenna branches has usually been performed on regular basis (once a day, for example).
Alternatively, there is the prior art of compensating amplitude and phase variations on each antenna branch by sending a pilot signal. FIG. 20 is a view for describing the prior art.
Circuits 110a through 110d each including various nonlinear elements (LNAs, mixers, and the like) are located on four transmission paths respectively. The antennas 101a through 101d are also located on these transmission paths respectively.
As shown in FIG. 20, pilot signal axc2x7exp(jxcex8) is sent to the antennas 101a through 110d from a direction so that phase differences will not arise. It is assumed that signals which are processed in and output from the circuits 110a through 110d are a1xc2x7exp(jxcex81) through a4xc2x7exp(jxcex84).
The value of the ratio of signal a1xc2x7exp(jxcex81) output from the branch on which the antenna 101a is located and signal a2xc2x7exp(jxcex82) output from the branch on which the antenna 101b is located is (a1/a2)xc2x7exp[j(xcex81xe2x88x92xcex82)]. By multiplying this value and the original signal a2xc2x7exp(jxcex82), which is output from the branch on which the antenna 101b is located, together, a1xc2x7exp(jxcex81) is obtained. That is to say, the amplitude and phase variation between the branch on which the antenna 110a is located and the branch on which the antenna 101b is located are compensated. This value therefore should be used as a compensation value for the branch on which the antenna 101b is located. This is the same with the other branches.
Conventionally, such a compensation value has been calculated for each antenna branch by the use of a pilot signal to compensate an amplitude variation and phase variation.
However, with the above conventional method in which calibration between antenna branches is performed on regular basis, beam forming will be performed on the basis of uncertain compensation conditions because of dynamic amplitude and phase variations. As a result, the reliability of a system is low.
On the other hand, with the above conventional method in which a pilot signal is used, a dedicated unit for generating a pilot signal must be located in all of the cells or sectors, resulting in a heavy economic burden.
Further, this pilot signal will become an interference source. In that case, a circuit for performing the above operation needs to be located in order to process a pilot signal, being an interference source. This will make a system inefficient.
The present invention was made under the background circumstances as described above. An object of the present invention is to provide a variation compensating unit for improving system reliability and communication quality by compensating amplitude and phase variation efficiently.
To achieve the above object, according to the present invention, there is provided a variation compensating unit for compensating variation including at least one of an amplitude variation and phase variation which will occur in the case of transmitting signals via transmission paths, the unit comprising: a first combining section for generating a first combined signal by combining signals on different transmission paths in which the variation has not occurred; a second combining section for generating a second combined signal by combining signals on the different transmission paths in which the variation has occurred; a compensation value calculating section for calculating compensation values for compensating the variation according to the transmission paths on the basis of the first combined signal, the second combined signal, and the signals on the different transmission paths in which the variation has not occurred or in which the variation has occurred; and a compensating section for compensating the variation dynamically on the basis of the compensation values.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.